docs: def _get_site_type_dummy_variables(self, sites):
          """
          Get site type dummy variables, ``Sc`` (for soft and stiff soil sites)
          and ``Sd`` (for rock sites).
          """
          Sc = np.zeros_like(sites.vs30)
          Sd = np.zeros_like(sites.vs30)
          # Soft soil; Vs30 < 360 m/s. Page 199.
          Sd[sites.vs30 < 360.0] = 1
          # Stiff soil 360 <= Vs30 < 760
          Sc[np.logical_and(sites.vs30 >= 360.0, sites.vs30 < 760.0)] = 1
  
          return Sc, Sd